How to distinguish the sex of rats? - briefly
Examine the anogenital distance—males have a noticeably larger gap between the anus and genital opening—and look for the presence of a scrotum or testes; females display a shorter distance, a vaginal opening, and lack visible testes. Additional cues include the shape of the genital papilla and the absence of male-specific scent glands.
How to distinguish the sex of rats? - in detail
Identifying the sex of rats requires careful observation of external anatomy, especially when the animals are young. The most reliable indicator is the presence or absence of a visible scrotum in males and the shape of the genital opening in females.
In adult specimens, the testes are located within a distinct scrotal sac that appears as a pair of pinkish bulges behind the hind limbs. The penis is hidden within the preputial fold and not easily seen without manipulation. Females lack a scrotum; instead, a single, round opening (the vaginal orifice) is situated between the anus and the base of the tail. The distance between the anus and the genital opening (anogenital distance) is shorter in females than in males.
For juveniles, sex determination is more challenging because the scrotum has not yet descended. The following steps improve accuracy:
- Gently restrain the rat to expose the ventral surface.
- Locate the anogenital region; in males, a small, raised ridge (the preputial sheath) may be visible.
- Measure the anogenital distance with calipers; a distance greater than 5 mm generally indicates a male, while less than 5 mm suggests a female (values vary with strain and age).
- If uncertainty remains, apply a mild anesthetic and examine the internal genitalia through a small incision, or use DNA-based sexing kits that detect the Sry gene.
Additional considerations:
- Age: after 21 days, sexual dimorphism becomes more pronounced.
- Strain: some strains exhibit delayed scrotal descent, requiring later assessment.
- Handling: use gloves and minimize stress to prevent injury and hormonal alterations that could affect genital morphology.
When breeding colonies are managed, routine sex verification at weaning (around 21 days) and again at sexual maturity (6–8 weeks) ensures proper group composition and prevents unwanted litters.